Weather Blog: Reason for extended cold snap

Thank you for stopping by the Weather Watch 12 Blog! March 2013 has resembled the heart of winter, versus a transitional month to spring. In today's Weather Watch 12 Blog we discuss why much below average temperatures continue to linger, and when a change in the overall pattern may occur.

One of the key components driving our cold weather right now in southeast Wisconsin is the Arctic Oscillation(AO). The AO refers to an opposing pattern of pressure between the Arctic and areas like North America and northern Europe. If the air pressure is high in the Arctic, it tends to be low in areas such as northern Europe and North America. If the air pressure is low in northern Europe or North America it is often high in the Arctic.

The pressure differences can lead to a positive or negative AO index.

Negative AO

High pressure in the Arctic and low pressure in northern Europe and North America

Positive AO

Low pressure in the Arctic and high pressure in northern Europe and North America

Air moves around the globe because of differences in air pressure. Air or wind flows from areas of high pressure to areas of lower pressure. When the AO is negative the air flow from high pressure to low pressure will result in cold air flowing south. In this negative phase cold air can spill into the United States.

When the AO is positive the air flow from high pressure to low pressure pushes warmer air north and keeps the cold air in the arctic.

The image below is courtesy of NASA and shows the two phases of the AO.

The AO has been in a negative phase since early February leading to surges of cold arctic air and also several stretches of active weather. The AO index is currently at it's lowest observed point on the chart below. The red line is a forecast for the AO index which has it nearing positive territory by around April 1. If this occurs temperatures may finally begin to moderate as arctic air retreats.

Another factor adding to our cold is the snow pack that exists from the Midwest to the North Pole. Cold air drifting south moves above a cold, snow packed surface preventing the airmass from moderating much as it tracks into the northern U.S.

Now that we have discussed the AO, let's look at the 500mb or mid-level flow of our current weather pattern. The 500mb chart below is from Tuesday, March 19 at 7am. Notice the huge blocking high near Greenland and the trough of low pressure over the Midwest. Remember above we mentioned that air flows from areas of high pressure to low pressure. In this case, very cold air is allowed to spill into the central U.S.

Now let's use the AO and combine it with our knowledge of the LRC, or Lezak's Recurring Cycle. With a cycle duration around 51-53 days we can look back to late January. Comparing March 19 with the pattern around January 25 shows the troughs moving through the pattern in the favored long term long wave position. Each part of the pattern delivered cold weather to southeast Wisconsin.

Expect more below average temperatures in our area until we move into a different part of the pattern and the AO trends a little closer to neutral(0). Along with the cold temperatures an active weather pattern will likely continue into the start of April.

Fast forwarding to the next cycle in May, we will need to watch for a late season frost/freeze as this part of the pattern returns around May 10.

For the latest weather information watch WISN 12 News, and for updates throughout the day follow me on Twitter @jnelsonweather

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